1. Field of the Invention
The present invention relates to a method of producing carboxylic acids or carboxylic acid esters, more specifically, relates to a method of producing carboxylic acids or carboxylic acid esters having their contents of fluorine compounds (inorganic fluorine compounds and organic fluorine compounds) reduced.
2. Description of the Prior Art
As one of the methods for producing carboxylic acid esters from olefins in the presence of a strong acid catalyst, Koch reaction has been known, in which an olefin is carbonylated with carbon monoxide to form an acyl compound, the acyl compound is converted into a carboxylic acid by the reaction with water, and the carboxylic acid is then converted into an ester by the reaction with an alcohol.
The productions of pivalic acid from isobutene and C6-C11 carboxylic acids from C5-C10 olefins are known as the examples of Koch reaction for producing secondary or tertiary carboxylic acids.
As the catalyst for the production of carboxylic acid by Koch reaction, an inorganic acid such as sulfuric acid, HF and phosphorous acid is used singly or in the form such as HF·SbF5 and H3PO4·BF3 in combination with BF3 or SbF5. In the later stage of the reaction, water is added to separate the product and the catalyst. Therefore, this method involves a problem that the catalyst diluted with water should be recovered and regenerated.
The esterification of a carboxylic acid with an alcohol is conducted while removing water in the presence of an acid catalyst, for example, an inorganic acid such as sulfuric acid, HF, phosphorous acid and hydrochloric acid; an alkylsulfonic acid such as methanesulfonic acid and trifluoromethanesulfonic acid; an arylsulfonic acid such as p-toluenesulfonic acid; a metal alkoxide such as tetrabutoxytitanium; or a Lewis acid such as boron fluoride etherate. Since the rate of esterification is low in case of esterifying a branched secondary or tertiary carboxylic acid, the esterification of such a carboxylic acid involves difficulty for its industrial use.
To solve such a problem, proposed is a method in which an olefin is carbonylated into an acyl fluoride in the presence of HF as an acid catalyst, and then the acyl fluoride is esterified with an alcohol in the presence of HF (for example, U.S. Pat. No. 5,463,095). The proposed method is excellent because the recovery of catalyst is easy, and the carbonylation and esterification proceed quite easily. However, the patent describes and considers nothing about the stoichiometric control of the acyl fluoride and alcohol and the reduction of the content of organic fluorine compounds in the product.
As a result of research in view of solving the problems in the above known methods, the inventors have found that a carboxylic acid ester can be efficiently produced from an olefin, carbon monoxide and an alcohol, and filed a patent application on the basis of this finding (JP 9-328451A). In the proposed method, the esterification in the presence of HF catalyst is conducted by controlling the addition amount of the alcohol, thereby regulating the content of residual acyl fluoride in the produced ester within the specific range, and preventing the formation of water due to the dehydration of non-reacted alcohol. After removing HF catalyst, the residual acyl fluoride is esterified with an alcohol. Since the formation of water which is difficult to separate from HF catalyst is prevented, the high performance HF catalyst can be recycled for reuse. In addition, the carboxylic acid ester having its content of fluorine impurities reduced can be easily and stably produced. Therefore, the proposed method is industrially advantageous. However, it was found that high boiling products were decomposed under heating during the purification of the carboxylic acid ester by distillation, resulting in the contamination of the carboxylic acid ester with fluorine impurities such as HF and acyl fluoride. The fluorine impurities reduce the purity of the products and cause troubles such as corrosion of apparatus.